A portable electronic communication device, such as a personal digital assistant (PDA), a mobile phone, or a smart phone, requires an antenna to establish a wireless connection with another device in the communication system. Mobile communication providers provide mobile communication services using a predetermined frequency band allocated thereto. Accordingly, mobile terminal manufacturers provide different antennas for the different frequency bands provided by the mobile communication providers. Therefore, in order for a mobile terminal that provides a communication service in a high frequency band to also provide a communication service in a low frequency band, the mobile terminal requires an additional mounting space for additional antenna length to also handle the low frequency band, and thus a change in the design of the mobile terminal is necessary. This increases costs to a mobile terminal manufacturer due to design changes and redevelopment of the antenna and also results in a corresponding increase in purchase price of the mobile terminal for a consumer.
The performance of an antenna will impact the communication range of a RF system. Since range is often a critical factor when designing RF systems, it is important to be able to characterize the antenna. One parameter that is important, and which can easily be measured is the return loss (RL). Impedance mismatch between the feeding transmission line and the antenna causes reflection at the feed point of the antenna. Because of this reflection not all of the available power will reach the antenna, and thus the field strength of the radiated signal will be reduced. RL describes how much of the available power is reflected at the feed point of the antenna.
PCB (printed circuit board) antennas are one example of antennas that are sensitive to surroundings, including such sensitivity factors as PCB material, layout of antenna element and/or ground plane element, other nearby electrical components, nearby metal materials, device housings forming antenna enclosures and so on. For example, two PCB antennas with the same size patterned as traces on different PCBs may demonstrate different performances. Even two identical PCB antennas may have two distinct resonant frequency values and input impedance values when used in different products. If the resonant frequency shifts out of band, the input impedance increases/decreases beyond tolerance or other performances beyond tolerance, the designer will encounter a big problem in designing and verifying procedures of the antenna.
When a PCB antenna designed for a specific product is tested and found that its resonant frequency is out of band, input impedance is beyond tolerance or other performances are beyond tolerance, the layout of the PCB antenna typically is redesigned to form a modified PCB antenna accordingly. The design and test procedures will be continuously performed until the modified PCB antenna passes the verification test. Besides, if the housing or the PCB material of the product is changed by manufacturers due to some reasons, it typically needs a PCB antenna of new version to fit the change of the surroundings, which is time consuming and cost effective. The condition becomes worse when a particular portable electronic communication device requires different antennas for different applications, e.g. cellular, GPS, Bluetooth and so on, some of which operate at different bands (i.e. differently configured antennas) depending upon the country of operation.
For a designer, adding a matching circuit to a feed pin of the PCB antenna without adjusting the layout of the PCB antenna is another practicable manner. However, there are only several specific matching circuits available in the markets and the properties of matching circuits are different based on different suppliers, such that the performances of the PCB antennas having different matching circuits are discrete. That is, the PCB antenna resonates at M frequency when a M matching circuit is added to the PCB antenna, and the PCB antenna resonates at N frequency when a N matching circuit is added to the PCB antenna. And the designer cannot make the PCB antenna operate at an arbitrarily frequency between M and N because a suitable matching circuit is unavailable.
There are several ways to tune an antenna to achieve better performance. For resonant antennas the main factor is the length. Ideally the frequency which gives least reflection should be in the middle of the frequency band of interest. Thus if the resonance frequency is too low, the antenna should be made shorter. If the resonance frequency is too high, the antenna length should be increased. Even if the antenna resonates at the correct frequency it might not be well matched to the correct impedance. Dependent of the antenna type there are several possibilities to obtain optimum impedance at the correct frequency. Size of ground plane, distance from antenna to ground plane, dimensions of antenna elements, feed point, and plastic casing are factors that can affect the impedance. Thus by varying these factors it might be possible to improve the impedance match of the antenna. However, all of these methods are time consuming in the design process.
Thus, there is a need for a method for adjusts the resonant frequency, the input impedance and/or other performances of a PCB antenna effectively and economically, and a structure thereof.